We have synthesized single-layer graphene on Cu foils using chemical vapor deposition method and transferred the graphene to the top of a Si/SiO2 substrate with a pair of prefabricated Ti/Au electrodes. A resistive graphene-based gas sensor prepared in this way revealed n-type oxygen response at room temperature and we have successfully fitted the data obtained with varying oxygen levels using a two-site Langmuir model. p-Type oxygen response of our sensor was observed after the temperature was raised to 100 °C, with a reversible transition to n-type behavior when the temperature was lowered back to room temperature. Such inversion of the gas response type with temperature was interpreted as a result of interplay between the adsorbate-induced charge transfer and charge carrier scattering. The transduction function was derived, which relates the electrical response to surface coverage through both the induced mobility and charge density changes.

Photo-induced changes in the electrical conductivity and the sensitivity to oxygen gas of graphene sheets grown by chemical vapor deposition and transferred onto Al2O3 and SiO2 thin film substrates were studied at ambient conditions. The pristine graphene sensors were initially completely insensitive to oxygen gas at room temperature but showed significant (up to 100%) response when illuminated with weak ultraviolet (300 nm or 365 nm) light. Oxygen response was governed by Langmuir law and its activation was insensitive to humidity. The mechanism of sensitization is analyzed together with other photo-induced effects—negative persistent photo-conduction and photo-induced hysteresis of field effect transistor characteristics. While the reduction of conductivity in air is persistent effect, the oxygen sensitization and enlargement of hysteresis take place only under the direct influence of light. It is concluded that the charge traps with differently adsorbed oxygen and water are involved in these phenomena.

New corrosion-resistant hybrid coatings based on graphene and polypyrrole were prepared and studied. A two-step process was used to fabricate the coatings. First, graphene was grown on a commercial copper foil by chemical vapor deposition (CVD) method. As the coating consisting only from graphene was imperfect, the defects were sealed with polypyrrole using electrodeposition. Corrosion resistance of copper, copper with CVD-graphene coating, and copper with CVD-graphene/polypyrrole hybrid coating were tested by immersion in salt water and also studied by open circuit potential measurements of the samples under ultraviolet radiation exposure, linear sweep voltammetry and cyclic voltammetry. The surfaces of the samples were also characterized by different microscopy and Raman spectroscopy methods. Although CVD graphene protected copper against corrosion in a certain extent, immersion tests revealed that the metal corrosion is mainly due to the graphene defects. The hybrid coatings showed much better anticorrosion performance. (C) 2014 Elsevier B.V. All rights reserved.

Responses of enzymatic bio-optrodes in flow regime were studied and an original model was proposed with the aim of establishing a reliable method for a quick determination of biosensor signal parameters, applicable for biosensor calibration. A dual-optrode glucose biosensor, comprising of a glucose bio-optrode and a reference oxygen optrode, both placed into identical flow channels, was developed and used as a model system. The signal parameters of this biosensor at different substrate concentrations were not dependent on the speed of the probe flow and could be determined from the initial part of the biosensor transient phase signal, providing a valuable tool for rapid analysis. In addition, the model helped to design the biosensor system with reduced impact of enzyme inactivation to the system stability (20% decrease of the enzyme activity lead to only a 1% decrease of the slope of the calibration curve) and hence significantly prolong the effective lifetime of bio-optrodes.

Amorphous granular SnO2 thin films were investigated from a standpoint of an NO2 gas sensor working at room temperature. The films were deposited using pulsed laser deposition method with substrate at room temperature and ∼90nm thick SnO2 films with amorphous structure were obtained as a result. SnO2 films deposited on Pt electrode substrates formed a sensor structure that showed response Iair/Igas to 4ppm NO2 up to ∼8000. I–V characteristics of the sensor structure were described by the power law dependence, whereas the power indexes were different for measurements in pure air and in the presence of NO2. As a result, the sensor response was highly dependent on bias voltage between the sensor electrodes. It was demonstrated that the nonlinear electrical characteristics and bias dependent gas sensitivity were the inherent properties of thin films and the contacts were ohmic.

In the present work, it is shown that carbon nanotube-doped transition metal oxides are potential candidates for use as ceramic transparent electrode materials. Used carbon nanotubes (CNT-s) are synthesized by using chemical vapor deposition (CVD) method. Electrodes in shape of fibers are obtained via inexpensive and low temperature sol-gel method. Due to extraordinary electrical and optical properties of CNT-s and good chemical and physical stability of metal oxide ceramics, resulting composites could be an interesting subject for industry.

A model is presented for describing the conductivity dependencies of p-type thin film sensors from oxygen and CO pressures. The model basis on the kinetic equations, composed for the processes of dissociative ionization of O2 and oxidation of CO on the surface, and taking into the account the charge balance between the adsorbed species and the hole accumulation layer. The model results are in agreement with the experimental data obtained on thin film sensors with two different p-type materials: Co3O4 and CoWO4. Both the experimental and model data differ essentially from the data obtained previously for n-type materials like SnO2. In particular, it was found that the sensitivity to CO increases significantly with decreasing O2 content in ambient environment. It is predicted that a similar effect (i.e. an increased sensitivity) can be obtained by engineering the intrinsic surface bending of the valence band.

Gas sensing properties of Co-oxide thin films were investigated and modelled. The films of cubic Co3O4 structure and with thickness between 20-150 nm were fabricated by pulsed laser deposition. The response of dc-conductivity to different gases (CO, NO2, CH4) was tested at temperatures 240 - 400 °C, and at different oxygen pressures. The conductivity changes due to O2 and due to test gas were found to be almost independent. The commonly used ionosorption model, which assumes reactions with adsorbed oxygen ions, failed to describe the dataset obtained at different CO and O2 pressures. Alternative mechanisms are discussed.

Pulsed laser deposition was applied to chromia-titania solid solutions (CTO) with the aim to study electrical and gas sensing properties on (ultra)thin films of this material. The films with different compositions (5% to 33.3% of Ti from total cationic amount) were characterized by XRD, XRR, AFM, and EPMA. The electrical conductivity of the films was studied in clean air and in the presence of reducing gases (H 2 , CO). The gas sensitivity was found to depend strongly on growth and annealing conditions but only slightly from thickness (10-70 nm) or composition of the films prepared under similar conditions.

The experience of working under an ISO 17025 compliant quality system in a university environment is described in the example of the Testing Centre of University of Tartu. University-specific features of the quality system include that the Testing Centre acts as the “quality system server” for other units of the university and graduate students participate in the work carried out under the quality system. The benefits for the university and the graduate students are discussed. It is concluded that implementation of an ISO 17025 compatible quality system in a university is fully accomplishable and gives significant added value to the university by helping to destroy the ivory tower perception about universities and by introducing real-world flavour into the studies and broaden the minds (quality awareness) of the students.

An essential factor for the creation of oxygen sensors based on luminescence quenching is the long-term stability of the parameters determining the luminescence decay. In the present work, the effects of material aging and photobleaching on decay parameters are studied for Pd-tetraphenylporphyrin and Pd-pentafluoro-tetraphenylporphyrin molecules embedded into different polymer films. In polymethylmethacrylate host the decay was well described by stretched exponent function. During annealing of PMMA films at different temperatures between 20 and 70 degrees C during 9 months a logarithmic decrease of the material sensitivity occurred due to the physical aging of the polymer. The photostability studies of PMMA films showed an opposite effect: the photobleaching was accompanied by a decrease in natural decay time, whereas. the oxygen sensitivity remained practically constant. Luminescence quenching in polystyrene and polycarbonate films had different character as compared to PMMA, and was interpreted with a two-site model, combined with a nonlinear gas transport model. In these polymer hosts the oxygen sensitivity increased during the first stage of aging, which was ascribed to the changes in the Langmuir component of gas transport, i.e., to the evaluation of microvoids in these glassy polymers.